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Chapter 2: Hardware information
IMPORTANT! When using PCI cards on shared slots, ensure that the
drivers support “Share IRQ” or that the cards do not need IRQ
assignments. Otherwise, conflicts will arise between the three PCI groups,
making the system unstable and the card inoperative.
2.6.2 Configuring an expansion card
Some expansion cards need an IRQ to operate. Generally, an IRQ must be
exclusively assigned to one function at a time. In a standard design
configuration, 16 IRQs are available but most are already in use. Normally,
6 IRQs are free for expansion cards. If the motherboard has PCI audio
onboard, an additional IRQ will be used. If your motherboard also has MIDI
enabled, another IRQ will be used, leaving 4 IRQs free. Sometimes IRQs
are “shared” by more than one function; in this case, IRQ assignments are
swapped automatically or adjusted through the BIOS firmware.
Standard Interrupt Assignments
IRQ Priority Standard Function
0 1 System Timer
1 2 Keyboard Controller
2 N/A Programmable Interrupt
3* 11 Communications Port (COM2)
4* 12 Communications Port (COM1)
5* 13 Sound Card (sometimes LPT2)
6 14 Floppy Disk Controller
7* 15 Printer Port (LPT1)
8 3 System CMOS/Real Time Clock
9* 4 ACPI Mode when used
10* 5 IRQ Holder for PCI Steering
11* 6 IRQ Holder for PCI Steering
12* 7 PS/2 Compatible Mouse Port
13 8 Numeric Data Processor
14* 9 Primary IDE Channel
15* 10 Secondary IDE Channel
*These IRQs are usually available for ISA or PCI devices.
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